Digital photographing devices traditionally comprise an image sensor, an autofocus (AF) system and an assist light that illuminates the object being photographed in low light conditions. CMOS (Complementary Metal Oxide Semiconductor) and CCD (Charge Coupled Device) are typical examples of image sensors. The operation between these sensors is different. For CCD sensor's each pixel reacts to light and stores the electrical charge caused by the light, which charge is further transmitted to be converted to a voltage. In the CMOS sensor, each pixel, on the other hand, is configured to convert the charge to voltage whereby the charge does not need to be transmitted anywhere. In addition to the charge-to-voltage conversion, a CMOS sensor may include AD conversion and some signal processing means as well. An image sensor is equipped with a shutter, such as e.g. a rolling shutter for CMOS sensor. A global shutter exposes entire image frame simultaneously, whereas the rolling shutter exposes different points of the frame at different points of time—in practice exposing the frame line by line.
The image sensor of the camera is configured to produce image data for the autofocus unit. Autofocus system is configured to move a lens of the device in and out until the image is in its sharpest form. The autofocus unit calculates parameters for the motor adjusting the position of the lens on the basis of the data. The assist light assists the photographing device's focusing system during capturing process. Because of power and heat generation reasons, the assist light cannot be high power.
This problem has been approached with solutions, where suitable current is drawn continuously or PWM (Pulse Width Modulation) dimmed to the assist light. The PWM reduces the total amount of power delivered to the assist light. However, when the assist light is powered with small amount of power, the light does not become as bright as the reliable autofocus would expect. Therefore an improved solution for more reliable autofocus is needed.